1. Field of the Invention
The invention relates to a self-pulsing multi-section laser provided with two DFB (distributed feedback) sections.
2. Prior Art
In recent years functional multi-section lasers have been developed for the regeneration of signals in ultra-fast optical telecommunication networks, for attaining, purely optically, higher operating velocities with simplified and more compact signal processing modules, while at the same time avoiding opto-electrical conversions.
In accordance with the state of the art, low-frequency self-pulsations in semiconductor lasers may be generated by bleaching out an absorber, e.g. in three section DFB (distributed feedback) lasers (ELECTRONICS LETTERS, Nov. 10, 1988, Vol. 24, No. 23, pp. 1426-1427) or in two section Fabry-Perot (FP) lasers (IEEE Photonics Technology Letters, Vol. 3, No. 10, October 1991, pp. 942-945). The physical limit of velocity is directly related to the lifetime of the charge carrier in the absorber section. By utilizing absorptive effects, the self-pulsating frequencies are, therefore, limited to several hundred MHZ up to several GHz. Furthermore, in order to achieve any self-pulsation at all, the lifetime of the charge carrier in the absorber must be less than that in the pump section, which requires special technological treatment of the absorber section.
Higher self-pulsation frequencies have been realized by utilizing dispersive effects in two-section DFB lasers in which the configuration of the individual sections does not require any additional effort.
The state of the art, from which the invention is proceeding, relates to a multi-section DFB laser provided with two DFB sections, and has been described in IEEE Photonics Technology Letters, Vol. 4, No. 9, September 1992, pp. 976 to 978. The two optically coupled DFB sections of the multi-section laser have identical layer structures, and in their longitudinal direction they are electrically separated by an etch moat. The structure of a fin wave guide is provided with quarternary active layers of a wave length of the 1.55 .mu.m band gap. The first order DFB gratings were holographically fabricated and transferred to the upper layer of the wave guide by a wet etching process. The facets of the two DFB sections were not coated. For generating self-pulsations in this two section DFB laser, certain critical spectral correlations of laser modes are required in the two DFB sections. These may be attained by the setting of certain combinations of currents in the two sections.
With the described two-section DFB laser, pulsation frequencies were attained which were higher by an order of magnitude than in the above mentioned lasers incorporating absorbers. In this arrangement, the pulsation frequency can be tuned entirely electrically by continuous adjustment of the operating currents in the two sections over a wide frequency range. The results of continuing work have been reported in Proc. of 14th IEEE Intern. Semicond. Laser Conf., Sep. 19-23, 1994, pp. 227-228. An optimization of the DFB laser structure was carried out by structuring its active layer as a multi quantum well and by realizing a greater coupling coefficient of the DFB grating. In a two-section DFB laser optimized in this manner, it was possible continuously to the tune the pulsation frequency between 12 and 64 GHz, by changing the operating current of only one DFB section, while maintaining the operating current of the other section constant.
The results presented thus far cannot, however, be reproduced at random. It is known that the phase position of the light reflected at a facet has a significant effect upon the spectral characteristic of a DFB laser, relative to the DFB grating. With facets produced by cleavage the resultant phase position of each DFB laser is an individual and technologically uncontrollable factor. Since to date high frequency self-pulsations have been demonstrated only experimentally with DFB lasers having at least one unbloomed facet, it has thus far been necessary to select components for attaining lasers of suitable pulsation characteristics. Even nominally identical lasers fabricated directly adjacent each other on a chip displayed very great differences as regards the generation and characteristics (e.g. frequency) of the self-pulsations.